The present invention relates to a semi-active suspension control system for automotive vehicles which is substantially composed of controllable vibration dampers, sensors for obtaining the information about the chassis performance required for the control, and electronic circuits for assessing the sensor signals and for generating damper-actuating signals.
The absolute chassis velocity is the predominant control variable for the assessment of comfort in respect of a semi-active suspension control system. The magnitude of the damping force which is required for great driving safety and major control comfort is responsive to the chassis velocity. However, this control variable is not developed as a direct measurement. It is common to measure the chassis acceleration and to calculate the chassis velocity by integration of these measured variables.
In addition, the damper speed is required to produce a semi-active suspension control. The damper speed is determined by measurement of the damper travel, that is the relative travel between wheel and chassis, and by differentiation of this value or by integration of the wheel acceleration which can be measured by means of acceleration transducers. There are a number of known difficulties and problems associated with the assessment of the various sensor signals and such integration and differentiation.
European patent application EP-A-0 341 883 teaches an analog-digital method and a corresponding device for determining by approximation the absolute chassis speed for an active or semi-active suspension control system. In this method, the output signal of a chassis-acceleration sensor is assessed for the control algorithm by way of an analog integrator which also contains a low-pass filter of first order. Digital filters are used for separating the constant-frequency and the low-frequency components. Further, the control system comprises relative-speed sensors, the output signals of which are assessed for switching on and off the damper force which is generated by variable vibration dampers. A semi-active control system of this known type necessitates sensors both for determining the absolute chassis acceleration and for determining the relative movement between chassis and wheel. Considerable expenditure is involved.
Also, EP-A-0 341 883 mentions a so-called Kalman filter. To be precise, this system, which has been described by Karl Brammer and Gerhard Siffling in the text-book "Kalman-Bucy-Filter" in the series Methods of Control Technology by the R. Oldenbourg publishing house Munich (Germany), Vienna (Austria), 1975, on pages 75 to 83 and which is referred to as "filter," is a deterministic observer which starts from the mathematic description of the real system and corrects the (model) variables or variables of state which are determined and being measurable not at all or only by entailing great effort to form estimated quantities by virtue of a value which takes into consideration the stochastic properties of system malfunctions or measurement errors, the estimated quantities being obtained by filtering of the difference between measurable output quantities of the real system and the calculated output quantities of the model. However, the application of such filters in real systems requires a great calculating capacity so that they do not appear to be suited for use in suspension control systems of vehicles in view of the required expenditure.
Further, so-called 1/4 vehicle models and full car models which mathematically describe the physical behavior of wheels and chassis of an automotive vehicle are e.g. known from "Proceedings of the Institution of Mechanical Engineers" International Conference on Advanced Suspensions, 24 to 25 Oct. 1988; The Institution of Mechanical Engineers, Birdcage Walk, London (Great Britain), pages 35 to 42.